1. Field of the Invention
The present invention relates to a light-emitting device and a manufacturing method of the light-emitting device.
Note that in this specification, the light-emitting device refers to a display device such as an organic EL display device or a light source (a lighting device) such as lighting or a sign (e.g., a traffic light or a warning light). Further, the device includes a light-emitting element.
2. Description of the Related Art
In recent years, research has been extensively conducted on a light-emitting device including a light-emitting element utilizing a self-emission electroluminescence (hereinafter referred to as an “EL element”). The light-emitting device has extremely excellent features such as very high response speed, low-voltage driving, and low-power consumption, and no need of backlight because of its self-emitting properties. Therefore, the light-emitting device has attracted much attention as a next-generation display device and has been widely used for various devices with a relatively small image display portion (for example, a portable information terminal).
As one kind of EL elements, an “organic EL element” is given, in the organic EL element, an organic layer including at least a light-emitting layer is interposed between a pair of electrodes. In order to increase emission efficiency, the organic EL element includes a layer to control flow of electrons or holes such as an electron-injection layer, an electron-transport layer, a hole-transport layer, or a hole-injection layer in addition to the light-emitting layer. Materials for these layers are roughly classified into low molecular (monomer) materials and high molecular (polymer) materials, and thin films of the low molecular materials can be formed by an evaporation method, for example.
Further, when the organic EL element is formed over a large-area substrate, it is easy to form a planar light-emitting device having a large area without variation in light emission (or with less variation in light emission), which is extremely difficult to be formed by point light sources typified by incandescent lamps or LEDs or linear light sources typified by fluorescent lamps. In addition, the organic EL element is estimated to have higher emission efficiency than incandescent lamps or fluorescent lamps, and therefore it has attracted great attention as a light-emitting element suitable for a next-generation lighting device.
As described above, the organic EL element has attracted great attention in a variety of fields; however, it has a problem in that rapid decrease in performance is caused once the element is exposed to moisture. Against such a problem, generally, a drying agent is provided between two substrates with which the organic EL element is sealed (in this specification, a substrate over which the organic EL element is provided is referred to as a “base substrate” and a substrate provided so as to face the base substrate is referred to as a “counter substrate”).
Further, in a method proposed in Patent Document 1, instead of the drying agent, a barium oxide film is formed on an inner surface of back glass (corresponding to the counter substrate in this specification) and used as a hygroscopic film.
Note that in this specification, a pair of electrodes and layers involved in light emission between the electrodes, which have a stacked-layer structure, are collectively referred to as a “light-emitting element”.